Embodiments of the inventive concept relate generally to semiconductor memory devices. More particularly, embodiments of the inventive concept relate to nonvolatile semiconductor memory devices and related methods of operation.
Semiconductor memory devices can be roughly divided into two categories based on whether they retain stored data when disconnected from power. These categories include volatile memory devices, which lose stored data when disconnected from power, and nonvolatile memory devices, which retain stored data when disconnected from power. Because nonvolatile memory devices retain stored data when disconnected from power, they are often used to store data that must be retained even when devices are powered down.
Examples of volatile memory devices include dynamic random access memory (DRAM) and static random access memory (SRAM). Examples of nonvolatile memory devices include electrically erasable programmable read only memory (EEPROM), ferroelectric random access memory (FRAM), phase-change random access memory (PRAM), magnetoresistive random access memory (MRAM), and flash memory.
In recent years, there has been an increase in the number of devices employing nonvolatile memory devices. As examples, nonvolatile memory devices are now used increasingly in MP3 players, digital cameras, cellular phones, camcorders, flash cards, solid state drives (SSDs), to name but a few. In addition, there has also been an increase in the overall storage capacity of nonvolatile memory devices, resulting in a tremendous amount of nonvolatile data storage in use today.
Flash memory is among the more frequently adopted forms of nonvolatile memory. It can be found in a wide variety of devices, including standalone applications such as memory cards, portable devices such as netbook computers, home electronics such as televisions, and others.
Most flash memories have one of two configurations, including NOR-type flash memory and NAND-type flash memory. In NOR-type flash memory, two or more cell transistors are connected in parallel to a single bitline, and data is stored using channel hot electron and erased data using Fowler-Nordheim tunneling. In NAND-type flash memory, two or more cell transistors are connected in series to a single bitline, and data is both stored and erased using Fowler-Nordheim tunneling.
Memory cells in a flash memory typically store one or more bits of data each. A flash memory cell capable of storing one bit of data typically has two threshold voltage states—one state representing a logical “1” and another state representing a logical “0”. A flash memory cell capable of storing two bits of data generally has four threshold voltage states representing logical “11”, “10”, “00”, and “01”. Similarly, a flash memory cell capable of storing three bits of data typically has eight threshold voltage states representing logical “111”, “110”, etc. Moreover, various techniques are currently being pursued to store four or more bits of data per memory cell.